In the construction of multi-cavity injection molds, each mold is provided with an injection nozzle, through which hot liquid plastic is injected into the mold. The injection nozzles are mounted in a relatively massive steel block known as the backing plate or base plate, and this block is in turn attached to a face plate in the injection molding machine.
Hot liquid plastic is forced through a network of connecting passageways or manifolds, by means of the injection cylinder in the injection molding machine, and eventually reaches the injection nozzles for the individual molds or cavities.
It is of course essential that the plastic shall remain as far as possible at the original temperature at which it leaves the injection cylinder, so that it remains in a molten flowable state when it is injected into the cavity. If there is a substantial temperature drop, by the time the plastic reaches the cavity, then the cavity may not be completely filled, resulting in wasteful rejects. In extreme cases the plastic may in fact freeze or set hard in the nozzle, leading to costly machine down time while the nozzle is removed and cleared.
The problem of temperature drop in the injected plastic is of course created largely by the very size of the molds themselves. The large masses of steel used to fabricate the molds, constitutes a highly efficient heat sink, and heat is readily transferred from the hot plastic into the steel blocks from which the molds are built. In order to overcome this, it has been standard practice for many years to incorporate electrical heating elements throughout the construction of the mold, and in fact the mold may be subjected to a fairly lengthy pre-heating process by means of these heating elements, before any attempt is made to start up the actual injection of plastic.
However, the heating of the molds by means of such electrical heating elements itself creates further problems. If the mold is maintained at too high a temperature, then the individual cavities may not be cool enough to produce a rapid freezing of the plastic as soon as the cavity is filled. Consequently, the cycling time for that particular mold may be too slow for efficient production, and consequently, while it is clearly desirable to maintain the main body of the mold throughout which the hot plastic is carried in passageways, within reason as hot as possible, it is also desirable to maintain the individual cavities at a much lower temperature.
For this reason, in the construction of such molds, it is customary to provide a two-part construction on the injection side of the mold, with one part or backing plate, incorporating the manifold system throughout which the hot plastic is carried to the individual nozzles, and another part or cavity support plate supporting the individual cavities, each of which is supplied by an individual nozzle. Even using this procedure, it is desirable to ensure that the nozzle while being hot enough to avoid an undesirable temperature drop in the liquid plastic, shall not be of such a nature that it conducts significant heat to the cavity to which it is connected.